Perform a ton of different actions through various plugins: Search the web, run terminal commands, perform calculations, search your Google contacts or Google Docs, and much, much more.

GNOME Do's biggest strength is its awesome plugin library. No other app launcher can compare. It has a few official plugins that search the web and other popular GNOME apps (Rhythmbox, GNOME Dictionary, Pidgin, and Tomboy, to name a few), as well as third-party plugins that fill in any voids the GNOME Do developers haven't tapped, like uploading photos to Flickr, searching Google Maps, connecting to SSH hosts, and so much more. Even though GNOME Do hasn't been updated in awhile, it's still the most popular app launcher out there, which means third parties will keep adding plugins that make it integrate with new apps and stay relevant.

As mentioned above, GNOME Do development seems to have stopped. Apart from that, the only thing we could wish to see was even more plugins, or maybe perhaps more preferences that let you create custom keywords a la Executor for Windows. It's also based on Mono, which some Linux users don't like because its based off Microsoft's .NET. Still, even without a ton of preferences, its plugin library makes it the most powerful launcher around.

Synapse is GNOME Do's most up and coming competition, featuring a Zeitgeist plugin that can do a lot of really cool things. It has a lot of the more basic plugins that Do has, like Rhythmbox, Dictionary, and so on, but its still fairly young, so GNOME Do still wins in terms of sheer features. If you don't like Do for any reason, though, Synapse would be one of the first places I'd look for a replacement.

Launchy, our favorite application launcher on Windows, is also available for Linux, and its just as great. It has a lot of preferences that let you tweak how you use the app, though it doesn't have quite as many plugins as GNOME Do—and it definitely doesn't integrate with Linux as well as GNOME Do does. But, if you're just using your app launcher for launching apps, it'll give you more control over how the program works, which is nice.

Lastly, Kupfer is a nice app launcher who's biggest strength is its speed. If you have a lower-powered machine, Kupfer is pretty lightweight, and will probably perform better than some of the options above. Like the others, it has a nice plugin library, though it can't match GNOME Do's. It's also pretty customizable, which if you're willing to do some work, can solve the problem of its ugly default interface.

Those are the best app launchers for Linux, though you may have your own favorites (not the least of which is probably the default "Run" command on your distro). Share your opinions with us in the comments.

Lifehacker's App Directory is a new and growing directory of recommendations for the best applications and tools in a number of given categories.